Packing element systems used on composite plugs are typically designed as individual components comprised of a rubber packing element and a back-up extrusion resistant component. For example, FIG. 1 illustrates a composite plug 10 in partial cross-section. The plug 10 has a mandrel 12 with cones 14 and backup systems 16 arranged on both sides of a packing element 18. Outside the inclined cones 14, the plug 10 has slips 20. As shown here, the slips 20 can be a conventional wicker-type slip typically composed of cast iron.
The backup systems 16 have several elements, namely a wedge ring 16a composed of a composite, a solid backup ring 16b composed of Polytetrafluoroethylene (PTFE), and a slotted ring 16c composed of a composite. When deployed downhole, the plug 10 is activated by a wireline setting tool (not shown), which uses conventional techniques of pulling against the mandrel 12 while simultaneously pushing against a push ring 13. As a result, the element system (i.e., packing element 18, cones 14, backup systems 16, and slips 20) is compressed along the axis of the mandrel 12. In particular, the slips 20 ride up the cones 14, the cones 14 move along the mandrel 12 toward one another, and the packing element 18 compresses and extends outward to engage a surrounding casing wall.
During the compression and extension of the packing element 18, the backup systems 16 control the extrusion of the packing element 18 so that the material does not overly extrude axially, which would weaken any resultant seal. The slips 20 are pushed outward in the process to engage the wall of the casing, which both maintains the plug 10 in place in the casing and keeps the packing element 18 contained.
As will be appreciated, the plug 10 and most of its components are preferably composed of millable materials because the plug 10 is milled out of the casing once operations are done, as noted previously. As many as fifty such plugs 10 can be used in one well and must be milled out at the end of operations. Therefore, having reliable plugs 10 composed of entirely (or mostly) of millable material is of particular interest to operators. As noted above, the solid backup rings 16b of the backup systems 16 are typically compose of PTFE or similar material. Such a material can cause problems during mill up of the tool 10, leaving a ring of material, tending to gum up, etc.
To deploy the plug 10 downhole, operators may need to pump the plug 10 along the wellbore. For instance, the plug 10 may be pumped down a horizontal section of a wellbore at extremely high pump rates that create a high fluid velocity across the plug 10. The high fluid velocity, which can be in excess of 50 ft./sec., may cause the element system (i.e., packing element 18, backup systems 16, slips 20, etc.) to pre-set while running in the wellbore. For example, should the plug 10 be stalled for whatever reason in the casing during run-in, the high velocity of fluid used to pump the plug 10 may flare out components of the backup system 16, expanding it like a sail and causing pre-setting of the element system 30.
As will be appreciated, pre-setting of the plug 10 can be catastrophic and may require operators to use coil tubing to drill up the pre-set plug 10, which can be very costly. Even though there is a risk of pre-setting, operators still want to run the plug 10 in the hole at higher rates because this reduces the rig time costs. In other situations, operators want to run the plug 10 at higher rates due to the extended reach of the well.
In addition, as the composite plugs are pumped downhole, the slips 20 have the potential to flare out due to high fluid velocities past them. The slips 20 can also incur physical damage while tripping downhole or mishandling of the composite plug. One solution to these issues has been to increase the break load on the upper slip 20, adding pins to the upper cone 14 and/or using yield bands. Still, even these mechanical fastening means can be prone to damage during run-in.
To prevent pre-setting, the element system (i.e., packing element 18, backup systems 16, slips 20, etc.) of the plugs 10 have also been designed with different geometries. Adhesives have been used to glue components together, or the components have been wrapped with a shrink fit. However, pre-setting still occurs, and the conventional element system has created limitations on the speed that a plug 10 can be run in the hole.
In other related aspects of plugs with packing elements, it is known in the art to use other types of anti-extrusion devices. For example, U.S. Pat. No. 8,167,033 discloses anti-extrusion rings that have hard segments surrounded by an elastic matrix. The hard segments expand to form a near solid ring of rigid material within the elastomeric matrix to prevent extrusion of the packing element.
Additionally, it is known in the art to embed slip type components in the packing element of a plug. For example, U.S. Pat. No. 2,194,331 discloses a plug having a packing element with embedded metal pieces that help securely engage in the casing.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.